Polymer colloids with dimensions in the range from 5 to 1000 μm are extensively used in ion-exchange and chromatography columns, in various biological and medicinal applications, as calibration standards, toners, coatings and supports for catalysts. In many of these applications, particle size and size distribution are of key importance. The preparation of monodispersed submicrometer-size polymer beads with pre-determined surface and bulk properties is a well-established procedure. By contrast, the synthesis of larger particles with a narrow size distribution is a synthetic challenge: it is either material-specific, or time-consuming (that is, it requires several stages), or it does not provide a sufficiently narrow size distribution of the resulting particles. Moreover, control of microbead shapes in conventional polymerization reactions is generally limited to the preparation of spherical particles.
Recent progress in developing new microfabrication techniques and microreaction technologies has raised new opportunities in reaction engineering. Microreactors provide high heat and mass transfer rates, safe and rapid synthesis and the possibility of the development of new reaction pathways too difficult for conventional reactors.
Typically, the preparation of polymer particles with assistance of microfluidic methods has been accomplished via a two-stage process. In the first stage, a monomer or a liquid polymer was emulsified to obtain droplets with a narrow size distribution. In the next stage, the resulting droplets were hardened in a batch (that is, non-continuous) process.
Fluid manipulation to form fluid streams of desired configuration, dispersions, and the like, for purposes of fluid delivery, product manufacture, analysis, to give a few examples, has a well established history. For example, monodisperse gas bubbles, less than 100 micrometers in diameter, have been produced using a technique referred to as capillary flow focusing. In this technique, gas is forced out of a capillary tube into a bath of liquid, the tube is positioned above a small orifice, and the contraction of flow of the external liquid through this orifice focuses the gas into a thin jet which subsequently breaks into bubbles via capillary instability.
Microfluidics is a field involving the control of fluid flow on very small scales. Typically, microfluidic devices include very small channels, within which the fluid flows, which may be branched or otherwise arranged to allow fluids to be combined with each other, to divert fluids to different locations, to cause laminar flow between fluids, to dilute fluids, or the like. Significant effort has been directed toward “lab-on-a-chip” microfluidic technology, in which researchers seek to carry out known chemical or biological reactions on a very small scale on a “chip,” or a microfluidic device. Additionally, new techniques, not necessarily known on the macro scale, are being developed using microfluidics. Examples of techniques being investigated or developed at the microfluidic scale include high-throughput screening, drug delivery, chemical kinetics measurements, as well as the study of fundamental questions in the fields of physics, chemistry, and engineering.
Microfluidic reactors show promising applications in combinatorial chemistry (where rapid testing of chemical reactions, chemical affinity, or microstructure formation are desired), biochemical and organic chemistry syntheses, rapid screening of catalysts, and synthesis of inorganic particles (e.g., silica or semiconductor quantum dots). Rapid heat and mass transfer, high yield and reproducibility lead to enhanced efficiency of existing chemical reactions and allows one to explore new reaction pathways that would be difficult in conventional reactors.
It would be very advantageous to provide a method for producing polymeric particles with pre-designed size, shape, morphology, and composition. Such particles could be used in many applications from drug delivery, cell research, flow cytometry, chromatography columns, catalysis, and calibration standards to mention just a few.